1. Field of the Invention
This invention relates to a process for producing bismuth tertiary amyloxide useful for producing composite oxides containing bismuth oxide in the CVD method, sol-gel method etc.
2. Description of the Prior Art
Bismuth layered composite oxides are useful as ferroelectrics such as SrBi2Ta2O9 and Bi4Ti3O12 and superconductors such as Bi2Sr2CaCu2O8. As the starting material for producing such film by the CVD method or sol-gel method, bismuth alkoxides or bismuth tris-xcex2-diketonates have been examined. These compounds should have high purity so as not to contain any alkali metals such as sodium and potassium. Out of many bismuth alkoxides, those which can be refined to high degrees substantially using sublimation and distillation are only bismuth tertiary butoxide (referred to hereinafter as Bi(OtBu)3) and bismuth tertiary amyloxide (referred to hereinafter as Bi (OtAm)3). Because these compounds are tertiary alkoxides, the exchange reaction thereof with other alcohols or xcex2-diketones is easy, and can serve as the starting material for a large number of useful bismuth alkoxides and bismuth xcex2-diketonates.
For refining or mass production, distillation is generally more preferable than sublimation. Since the melting point of Bi(OtBu)3 is as high as 150xc2x0 C., it will be pyrolyzed if distilled at the melting point or more. Accordingly, Bi(OtBu)3 must be refined by sublimation, but efficiency is deteriorated. On the other hand, Bi(OtAm)3 has a melting point of 90xc2x0 C., and can be refined by distillation. Accordingly, there was demand for a process for producing a highly purified product of Bi(OtAm)3.
M. A. Matchett, M. Y. Chiang and W. E. Buhro, in Inorg. Chem. Vol.29,360 (1990), have disclosed a process for producing Bi(OtAm)3, which comprises reacting dimethylaminobismuth Bi(N(CH3)2)3 with tertiary amyl alcohol HOC(CH3)2C2H5 (referred to hereinafter as tAmOH) and then recovering the product in at least 90% yield by sublimation. Certainly, this prior art process is a process for producing high-purity Bi(OtAm)3. However, there is the problem that the yield of the starting material Bi(N(CH3)2)3, which is produced by reacting bismuth chloride with dimethylaminolithium, is as low as up to 50%. Further, Bi(N(CH3)2)3 is spontaneously flammable, is poor in thermal stability, and problematic in storage etc., and thus it is not preferable as the starting material for mass production of Bi(OtAm)3.
M. C. Massiani et al., in Polyhedron Vol. 10,437 (1991), have disclosed that a solution of sodium tertiary butoxide in tetrahydrofuran (referred to hereinafter as THF) is reacted with a solution of bismuth bromide in THF, whereby 3.1 g of Bi(OtBu)3 is recovered in 80% yield by sublimation. However, M. C. Massiani et al. do not refer to Bi(OtAm)3. Further, they do not refer to the purity of the resulting Bi(OtBu)3. Nor do they refer to a method of removing sodium bromide (NaBr) produced as a byproduct.
In the case of mass production, the removal of NaBr produced as a byproduct constitutes an important procedure. When the amount of NaBr is small, Bi(OtBu)3 can be recovered by sublimation directly from the NaBr-containing product, but when its amount is large, recovery is difficult.
As can be seen from the foregoing description, there was no process suitable for mass production of Bi(OtAm)3.
The present inventors attempted application of the production process of M. C. Massiani et al. to production of Bi(OtAm)3. A solution of sodium tertiary amyloxide (referred to hereinafter as NaOtAm) in THF was reacted with a solution of BiBr3 in THF, and crystals of NaBr produced as a byproduct were attempted to be separated by filtration through filter paper No. 131, but the fine crystals passed through the filter paper, and thus a transparent filtrate could not be obtained. Even if conditions such as reaction temperature, aging time, stirring rate etc. were changed, good results could not be obtained. In the pure THF solvent, particles of crystalline NaBr did not grow to be filtered off.
Further, the present inventors found the problem of discoloration; that is, colorless Bi(OtAm)3 rapidly turns violet due to an unknown reason, possibly because the pure THF solvent acts strongly on Bi(OtAm)3 or because THF can contain oxygen or water dissolved therein. From these results, it was found that the use of pure THF as the solvent in the process has problems such as difficult separation of the byproduct NaBr by filtration and easy deterioration of Bi(OtAm)3.
The object of this invention is to provide a process suitable for mass production of Bi(OtAm)3, to further provide a process of facilitating separation of crystals of a byproduct NaBr or KBr, or NaCl or KCl, through filtration after reacting NaOtAm or KOtAm with BiBr3 or BiCl3, and to further provide a process with less deterioration of Bi(OtAm)3.
According to one aspect of this invention, there is provided a process for producing bismuth tertiary amyloxide, which comprises reacting bismuth bromide or bismuth chloride with sodium tertiary amyloxide or potassium tertiary amyloxide in an organic solvent containing tetrahydrofuran, then separating a byproduct sodium bromide or potassium bromide, or sodium chloride or potassium chloride, by filtration and recovering bismuth tertiary amyloxide by distillation under reduced pressure.
According to another aspect of this invention, there is provided a process for producing bismuth tertiary amyloxide, wherein the organic solvent containing tetrahydrofuran is a toluene solution containing 5 to 30 weight % tetrahydrofuran.
In the basic reaction of this invention, the process for producing Bi(OtBu)3 , as disclosed by M. C. Massiani et al., has been applied to production of Bi(OtAm)3, and this reaction is expressed by each of the following two reactions:
BiBr3 (or BiCl3)+3NaOtAmxe2x86x92Bi(OtAm)3+3NaBr (or 3NaCl)xe2x80x83xe2x80x83(1)
BiBr3 (or BiCl3)+3KOtAmxe2x86x92Bi(OtAm)3+3KBr (or 3KC1)xe2x80x83xe2x80x83(2)
Hereinafter, the invention is described by reference to the process where NaOtAm was used.
This reaction is conducted in an organic solvent containing THF. The organic solvent is preferably an aromatic hydrocarbon, particularly preferably toluene. Hereinafter, the reaction using a THF-containing toluene solution is described.
The solvent is a mixed solvent consisting of a solvent for dissolving or suspending BiBr3 and a solvent for dissolving or suspending NaOtAm. The weight % of THF in the THF-containing toluene solution in this invention is defined as follows:
THF weight %={THF weight/(THF weight+toluene weight)}xc3x97100 (%)
This is indicative of the weight % of THF in the mixed solvent just after the reaction is finished, in other words, in the mixed solvent to be subjected to the filtration step.
In this invention, the THF-containing toluene solution is preferably toluene containing 5 to 30 weight % THF. If the content of THF is less than 5 weight %, BiBr3 and NaOtAm are hardly dissolved thus resulting in a reduction in the rate of reaction. On the other hand, when the content exceeds 30weight %, NaBr produced as a byproduct hardly grows as crystals, thus making separation thereof by filtration difficult. Further, Bi(OtAm)3 is deteriorated in the same degree as in pure THF, and therefore a content of THF outside of the above-defined range is not preferable.
The starting material NaOtAm is obtained by reacting Na with an excess of tAmOH at 100 to 130xc2x0 C. After unreacted tAmOH is distilled away, the remaining NaOtAm is dried and then dissolved in a THF-containing toluene solution.
Anhydrous BiBr3 or BiCl3, which is equimolar to the above NaOtAm, is dissolved in a THF-containing solution and then added dropwise to while being stirred, and reacted with, the THF-containing toluene solution of NaOtAm. The reaction temperature is 30 to 60xc2x0 C., and the reaction time is 0.5 to 5 hours or so. Then, the reaction mixture is aged by stirring it at 80xc2x0 C. for 6 hours.
The reaction mixture is cooled to room temperature and then filtered. This filtration is conducted in nitrogen or argon free of oxygen or humidity. The filtration method makes use of filtration under reduced pressure, filtration under pressure, centrifugal filtration etc. In the case of filtration under reduced pressure, filter paper NO. 131 (3 xcexcm pores) can be used to obtain a transparent filtrate. From the filtrate, the solvent is distilled away at 80xc2x0 C. at a pressure of 1 Torr, and then the residue is charged into a still pot where it is distilled at 120xc2x0 C. at a pressure of 1 Torr, to give white Bi(OtAm)3 crystals.
When the solvent is pure THF, NaBr or NaCl crystals pass through the filter paper, and the filtrate is not transparent and causes the filter paper to be clogged finally, thus making filtration or washing of the filter cake to the end difficult. If normal filtration is infeasible, Bi(OtAm)3 contaminated with NaBr or NaCl is liable to bumping, distillation thereof cannot be conducted steadily, and Bi(OtAm)3 recovered by distillation has been contaminated with NaBr or NaCl, thus failing to provide high-purity Bi(OtAm)3.
The yield of Bi(OtAm)3 obtained in Example 1 was 70% or more, and its analysis revealed that the content of Bi was 44.0wt % which almost agreed with the theoretical content of 44.4%, while the content of each of impurities such as Na, K, Mg, Ca and Fe was 1 ppm or less, indicating that this product had high purity.